Plot summary
Wu and Mako help evacuate Republic City, and Asami and Varrick build a fleet of flying mecha-suits to defend it. But Kuvira's forces attack earlier than expected. Her superweapon, now mounted on a giant mecha, devastates the Republic's navy, forcing Raiko to surrender. Korra and the airbenders capture the mecha's creator, Baatar Jr. After Korra threatens to keep him from Kuvira forever, he attempts to convince Kuvira to end the attack, but she has the mecha fire on his position, in an attempt to take out the Avatar and her allies.
Credits
Adapted from Wikipedia contributors, The Legend of Korra season 4. HTML formatting and reference markers removed.